A vesicle has a structure covered with a lipid bilayer membrane, and among such vesicles, an exosome is known as a vesicular granule which is present in body fluid of a living organism. Similar to a common cell surface, it is known that various membrane proteins are present on a surface of an exosome. Meanwhile, it is also found that microRNA (miRNA) is contained in the inside of an exosome in addition to various proteins such as cytokine.
It is also known that an exosome is secreted from various cells such as cells of an immune system or various cancer cells. Attention is given to the function of an exosome as a mediator for intercellular communication in a living organism and also to the relationship between an exosome and a physiological phenomenon or a disease such as cancer, and therefore a study thereto currently being made. For example, it has been already reported that, when an antibody of EpCAM as a cancer marker is used, an exosome is separated from circulating blood of a patient with ovarian cancer and a relationship is found between an expression amount of miRNA derived from exosome and progress of ovarian cancer (Non Patent Literature 1).
As a 4 transmembrane membrane protein expressed on an exosome, there are CD9, CD63 and CD81 belonging to tetraspanin family, and it has been reported in Non Patent Literature 2 that the amount of exosome is higher in plasma from a patient with melanoma compared to that of a healthy person, and it can be detected and quantified by an antibody against CD63 or an antibody against Caveolin-1 as a cancer-related marker. Furthermore, by combining and reacting a plasma sample after centrifuge with CD63 antibody or an antibody against various membrane proteins or the like, a signal derived from an exosome of a cancer patient is quantified and analyzed (Patent Literature 1).